Color discrimination is performed in the Drosophila retina by the coupling between the inner photoreceptors R7 and R8. Two distinct sets of ommatidia, which are distributed stochastically throughout the retina, are characterized by the specific expression of distinct rhodopsin genes (rh). The p ommatidia are characterized by the expression of rh3 in R7 and rh5 in R8, while the y ommatidia have rh4 in R7 and rh6 in R8. Expression of rh3 and rh4 in R7 is mutually exclusive, and the coupling between R7 and R8 rh's is absolute. The applicants are investigating the control of this exclusion and the factors that contribute to coordinate expressions of rh's. The specific aims of the proposed experiments are: (1) that Pax-6, a gene that plays a critical role in eye development in all species, is an essential regulator of rh gene expression and that the ancestral role of Pax-6 was as a regulator of late photoreceptor determination and the control of rh expression; (2) regulatory sequences will be sought that determine the correct expression of rh's in the particular subtypes of R7 and R; (3) factors that regulate rh expression will be identified using the yeast one-hybrid system, and the genes encoding these factors, in particular the orthodenticle gene, will be characterized genetically; and (4) an enhancer trap screen that relies on specific expression patterns in subsets of inner photoreceptors will be used to identify genes important for proper retinal patterning and color vision. This will allow us to understand how patterning of sensory receptor exclusive expression is achieved, addressing a problem that is common to other sensory systems.